Boiler slagging (i.e., the depositing of ash on convective surfaces) may cause fouling issues in the convective pass of coal-fired power plants and remains a significant issue to many utility companies. The problem is often initiated in a particular locus of the inlet cross section because of temperature and O2/CO imbalances. This is especially true for tangential coal-fired boilers designed for Eastern bituminous coals that are now burning coals with constituents that cause them to have lower ash softening temperatures. For such boilers and fuels, which are already likely to operate with fouling issues, installation of conventional low-NOx burners may exacerbate fouling issues by a substantial degree. This often results in the need to operate at low loads periodically to “drop slag,” which may cause a loss in revenue to the power plant. Further, increases in fouling may result in tube leaks and repair expense therefor, or in forced outages to clean the convective pass of the collected slag. Current slag control generally is a reactive process, with the focus upon attempting to clean/control the result of poor balance and distributions within the system.
In general, a tangentially-fired boiler furnace has four to nine levels of burners that inject fuel and air from each corner at a tangent to an imaginary circle drawn within the boiler. The original designers of these boilers assumed that the resulting fireball would be a homogeneous structure. However, this desired result has not been achieved in conventional systems, and the reasons for this are several. First, the air supply to the burners is regulated for the four burners on each level as a group, i.e., there is no separate air supply control for each individual burner. Second, fuel supply to each burner is inconsistent as flows tend to vary from burner pipe to burner pipe because of the nature of the fuel distribution system. These two factors lead to imbalances in the delivery of air and fuel. The result is that instead of a homogeneous burning mass, the burner array produces a series of burner flow fields that resemble an intertwining series of rising helixes, as discussed in more detail below.
Because of the air and fuel supply inconsistencies, velocities and temperatures in individual flow fields that develop often differ. Stoichiometries may vary as well, with the result that some flow fields are fuel lean, while others are fuel-rich. These imbalances often create conditions in which ash softening occurs in the convective section, which causes the depositing of ash on the convective surfaces. More specifically, a fuel-rich flow field (i.e., reducing atmosphere) may reach an ash softening temperature at a significantly lower temperature than a balanced or fuel lean flow field, thus increasing the likelihood of ash softening (and slag formation) in the convective section of the boiler. Temperature imbalances further mean that high temperature zones exist, which further increases the likelihood that the ash softening temperature is reached and slag forms.
Conventional systems have no ready means to diagnose or address this problem. This is particularly true in boilers designed for Eastern Bituminous coal that are now burning Western coals such as PRB. The problem is further exacerbated with the installation of conventional low-NOx burners, which operate at even lower average stoichiometries in the main combustion zone.
At present, boiler operators pay little heed to the balance of stoichiometries and temperature and their effect on slagging. Most operators, specifically on tangentially-fired boilers, have come to accept the imbalances as being “normal” for the type of boiler. Current slag control, therefore, becomes substantially a reactive process, with the focus upon attempting to clean/control the result of poor balance and distributions. As described, addressing slagging issues in this manner is inefficient and costly. Further, as one of ordinary skill in the art would appreciate, stoichiometry imbalances within the boiler cause system inefficiencies.
Thus, there is a demonstrated need for a system and method for proactively mitigating slag formation or fouling in boilers, especially tangentially coal-fired boilers. A system and method that achieved this goal while also increasing boiler efficiency would be particularly valuable to boiler operators. One such system may prevent or significantly reduce slag formation and increase efficiency by addressing the flow field imbalances that occur in conventional systems throughout the furnace. As described, when present, flow field imbalances lead to stoichiometric and temperature imbalances in the convective section of the boiler such that temperatures above ash softening points are experienced and ash is deposited on convective surfaces. There is a need for such a system to operate without sacrificing the NOx reductions made possible by the enhanced staging capabilities of the low NOx firing configuration.
Further, conventional set-up of tangential coal fired plants make the avoidance of such flow field imbalances within the furnace potentially difficult and costly. As such, there is a need for an improved system and method that is effective at avoiding such imbalances while being simple, such that it may be implemented in a cost effective manner in new boilers and/or retrofitted in existing boilers. It has been discovered that such a system and method may utilize effective zonal monitoring to drive a limited number of air injector nozzles in the upper furnace so as to mitigate zones of both high temperature gas and zones of fuel-rich flow fields prior to their entry into the convection pass where slag formation may occur.